1. Field of the Invention
The present invention relates to a radio communication system and, more particularly, to a radio communication system which is comprised of a plurality of base stations and a plurality of mobile stations that can be connected to the base stations through radio channels and controls the transmission outputs of polling signals from the base stations to allow the base stations to transmit a plurality of types of polling signals having different transmission outputs, for example, to each other, at predetermined timings.
2. Description of the Prior Art
For example, in the conventional radio communication system disclosed in Japanese Unexamined Patent Publication No. 8-107382, each base station controls the transmission output. According to the characteristic feature of this system, when a mobile station connected to a given base station or located in a radio zone formed by a given base station receives a signal from another radio zone, the transmission output of the base station or mobile station as the destination of the signal from another radio zone is reduced through a private communication network.
FIG. 1 shows an example of the arrangement described in the above reference. Base stations B.sub.i and B.sub.i+1 connected to each other through a private communication network respectively form radio zones Z.sub.i and Z.sub.i+1. Mobile stations T.sub.j and T.sub.j+1 are performing radio communication with the base station B.sub.i upon determining radio channels. A mobile station mobile station T.sub.j+2 is performing radio communication with the base station B.sub.i+1 upon determining a radio channel.
Since the radio zone formed by the base station B.sub.i+1 is large, the mobile station T.sub.j+1 receives a signal from the base station B.sub.i+1 as well. At this time, the mobile station T.sub.j+1 processes the signal from the base station B.sub.i+1 to identify and specify the base station B.sub.i+1 as a transmission source, and makes a request to reduce the transmission output of the base station B.sub.i+1 through the base station B.sub.i and the private communication network. In response to this request, the private communication network reduces the transmission output of the base station B.sub.i +, and reduces the radio zone formed by the base station B.sub.i +, as shown in FIG. 1.
With this transmission output control, each base station or mobile station can construct a satisfactory communication environment without being interfered by any signals from other adjacent radio zones.
As a radio communication system in which a base station polls a plurality of mobile stations to collect data, for example, the system disclosed in Japanese Unexamined Patent Publication No. 2-15740 or 6-29897 is available. According to Japanese Unexamined Patent Publication No. 2-15740, a base station communicates with all mobile stations to collect data, and sequentially collects data. For example, this system can be applied to collection of data from all mobile stations, e.g., collection of questionnaire.
The system disclosed in Japanese Unexamined Patent Publication No. 6-29897 has a characteristic feature in that a base station transmits a polling signal with a polling start number and a time slot count being added thereto. Assume that mobile stations T(1), T(2), . . . , T(M) are present in the radio zone formed by a base station, and the base station transmits a polling signal with a polling start number N (1.ltoreq.N.ltoreq.M) and a time slot count n (1.ltoreq.n.ltoreq.M-N+1) being added thereto. In this case, only mobile stations A(N) to A(N+n) respond to this polling signal. With this operation, a plurality of mobile stations can be managed in units of small groups.
In the scheme disclosed in Japanese Unexamined Patent Publication No. 8-107382, to prevent communication interference between adjacent radio zones, the transmission output of a communication station (base or mobile station) located in one radio zone is reduced. This conventional scheme is effective when a combination of one base station and a plurality of mobile stations belonging thereto is almost fixed, and the mobile stations do not move to other radio zones. If, however, such a mobile station moves between a plurality of base stations, when the mobile station receives a signal from another base station or mobile station, it is not easy for the mobile station to identify the signal as a signal from a radio zone other than the zone in which the mobile station is located. In practice, it is difficult to control the transmission output.
Consider a case wherein the base stations B.sub.i and B.sub.i+1 form adjacent radio zones Z.sub.i and Z.sub.i+1, and the mobile station T.sub.j+1 is located near the periphery of the radio zone Z.sub.i+1 and forms a radio zone Z(T.sub.j+1). The mobile station T.sub.j belonging to the base station B.sub.i receives a signal from the radio zone Z(T.sub.j+1), and hence makes a request to reduce the radio zone formed by the mobile station T.sub.j+1 through the base station B.sub.i and a private communication network. As a result, a transmission signal from the mobile station T.sub.j+1 cannot reach the base station B.sub.i+1, and data communication with the base station B.sub.i+1 cannot be performed.
In the radio communication system disclosed in Japanese Unexamined Patent Publication No. 2-15740, even when data needs to be transmitted to a specific mobile station, all the mobile stations in the system must be polled, resulting in a very long processing time. As an example of the countermeasures against this problem, the radio communication system disclosed in Japanese Unexamined Patent Publication No. 6-29897 is available. In this system, mobile stations are sequentially polled in groups by using idle time for processing other than polling. In this case as well, it takes a very long processing time to poll all the mobile stations.